Frequency stabilization systems



Dec. 6, 1955 M. ARDITI ET AL FREQUENCY STABILI-ZATION SYSTEMS 2 Sheets-Shee'fI l Filed Feb. 28, 1952 MGP..

NQS

United States I )atCflf O 2,726,332 FREQUENCY STABILIZATION SYSTEMS Maurice Arditi, Clifton, N. J., and Philip Pai-zen, Forest Hills, N. Y., assignors to International Telephone and Telegraph Corporation, a corporation of Maryland Application February 2s, 1952,'serial No. 273,846 11 claims. (ci. 25o- 36) 'Ihis invention relates to frequency stabilization'and more particularly to microwave frequency stabilizlng systems employing electron frequency converter-amplitiers.

One of the objects of this invention is lto provide a means of attaining systems having a high degree of -frequency stabilization in the microwave region. To achleve such systems, we employ electron frequency converteramplifiers which, at least in part, follow the principles of the traveling wave electron discharge device. The principles and adaptations of the traveling wave electron discharge device of the amplifier type for microwave frequency conversion are disclosed in the copending application of A. G. Clavier, M. Arditi, and P. Parzen, Serial No. 269,306, tiled January 3l, 1952, entitled Frequency Converter-Amplifiers. Therein is disclosed a wide band microwave frequency converter-amplifier employing a traveling wave electron discharge device wherein the beam forming electrodes and/or the slow-wave conductor or helical electrode thereof are utilized to modulate the projected electron beam. The electron beam forming electrodes comprise a cathode electrode, a focusing electrode, and an accelerator electrode. As disclosed in the aforesaid copending application, a modulating signal potential for beam modulation may be applied to any of the electrodes, including the helical electrode, or to any two or three of the four electrodes or betweenl any two of the four electrodes in such a manner that a phase modulation of the electrons of the beam is effected.

Employing the above-mentioned features of the frequency converter of the aforesaid copending application, we combine two frequencies, one by application to the R. F. section of a traveling wave tube and the other by application to the aforementioned electrodes for modulation of the electron beam. The modulated beam and the R. F. propagated wave interact in such a manner that a desired frequency may be selected by employment of v the proper R. F. band pass filter and passed to an appropriate antenna for radiation therefrom.`

An important feature of this invention is the utilization of the traveling wave converter-amplifier to control and stabilize the frequency of a radiated microwave signal. Besides stabilization of microwave signals, the traveling wave tube converter-amplifier provides for simplification of the usual automatic frequency control circuits, such as may be used with klystrons or other such microwave oscillators.

Frequency stabilization systems may be achieved by emplOyment f 'a feedback path originating at the output of the R. F. band pass filter, said path incorporating a means to correct any error in the transmitted frequency by producing a voltage which alters the intermediate oscillatory energy. In such an instance, the converter-amplifier will comprise. a .portion of-the feedback loop. Another system which may be employed incorporates a feedback loop substantially as described abovev with the addition of a conductive path originating from the output of they con- 1verter-amplifier for developing the energy of the vmicroy, 2,726,332 l latent-ed Dec. 6, 1955 ICC wave or radio frequency oscillatory source incorporating the converter-amplifier as a portion of the conductive loop. A system similar to the first above-mentioned system provides a feedback path to correct any error inthe transmitted frequency by producing a voltage which alters the R. F. oscillatoryenergy.v Further systems incorporate stabilization of the radio frequency and/ or the intermediate frequency oscillatorysource, with these feedback loops independent of the traveling wave converteramplifier. f v

Another feature of this invention is to provide a means of operating microwave oscillators and terminal transmitter equipment at a fixed frequency, no longer being dependent on the power mode operation of a klystron oscillator which heretofore produced inherently narrowfrequency bands. By operating microwave oscillators at a xed frequency and utilizing the video signal applied thereto in conjunction with the traveling wave Ytube converter-amplifier, the overall result ofthe terminal transmitter equipment will now be inherently wide. band with good linearity and capable of highpower outputs.

Still another feature of this invention is to utilize a traveling wave converter-ampliieras a microwave oscillator with provisions for a sharply-tuned cavity resonator to act as the feedback circuit. The modulation signal for this type oscillator being supplied'from an l. F. oscillator whose frequency is varied by employing a'voltage from an R. F. reference cavity discriminator which is coupled to the transmitter output, the variations therein causing an error voltage to be applied to the I. F. oscillator to correct the transmitted frequency.

Another feature of this invention is to provide a means of operating microwave oscillators in terminal transmitter equipment at a stabilized frequency wherein the modulating or intermediate frequency employed for interaction with the microwave signal from a microwave oscillator is derived from a stabilized sub-carrier system modulated from a video signal source. The resultant output of the terminal transmitter equipment will be inherently wide band with good linearity and capable of highV power outputs.

Another important feature of this invention is to utilize the traveling wave converter-amplifier as a traveling wave oscillator wherein the transmitted frequencyis stabilized by employing an R. F. reference discriminator which is coupled to the transmitter output,'variations therein causing an error voltage to be applied to an R. F. oscillatory source to correct the transmitted frequency. Provisions are made to modulate thistraveling wave oscillator by modulating the I. oscillator from a desirable signal source. f The above-mentioned and other features and objects of this invention will become. more apparentby reference to the following description taken in conjunction with the accompanying drawings, in which: i

Fig. l shows a longitudinal sectional view of a traveling wave electron discharge device and blocks illustrating the use of a converter-amplifier for the stabilization of a microwave oscillator frequency wherein one of the nuymerous methods of modulating the electron beam is indi- Referring to Fig. 1the utilization of theV traveling wave electron converter-amplifier device is illustrated with its accompanying D. C. elementary source, intermediate frequency section with its R. F. reference discriminator, and oneof the various methods of application of the intermediate frequency to interact with the R. F. frequency being propagated along a slow-wave propagating electrode. This systemas indicated adapts itself to stabilization of a microwave oscillator transmitted frequency.

The traveling waveelectron converter-amplifier 1 cornprises an electron gun section 2 and a radio frequency section 3. Electron gun section 2 includes the filaments 4, the cathode electrode 5, the focusing electrode 6, and the accelerator electrode 7, the latter three electrodes comprising the beam forming electrodes. Radio frequency section 3 includes the radio frequency input means 8, and the slow-wave propagating electrode 9, such as a helix, supported by end supports 10, the radio frequency output means 11, and the collector element 12. In .the electron gun section 2 an electron beam is produced which passes axially of electrode 9, the beam being confined axially by the magnetic field of the coil 3a of the R. F. section 3. This slow-wave propagating electrode 9 is for transmisysion of microwave energy for interaction with the closely associated electron beam. The characteristic of the slow-wave conductor 9 is such that the axial velocity of microwave signals conducted along the transmission path is approximately the same as or slightly slower than the velocity of the electrons of the beam, whereby the magnetic field of the microwave signal interacts with the electron beam for amplification of the microwave signal. The frequency conversion operation is accomplished by phase modulating the electron beam, as disclosed .in the aforesaid pending application. The interaction between the modulated electron beam4 and the microwave signal is such that a simultaneous frequency conversion and amplification is accomplished.

The embodiment shown in Fig. l illustrates that the I. F. signal is applied through the network 13 and is impressed between the cathode electrode 5 and the focusing electrode 6 for modulation of the electron beam. Elementary potentials may be established in substantially the manner as indicated by D. C. source 14. The elementary voltages with respect to ground that may be present on the various elements from such a source are: ground potential foraccelerator electrode 7 and slowwave propagating electrode 9, +150 volts for collector electrode 12, -3300 volts for the cathode electrode 5, and -335O volts for the focusing electrode 6. The application of the D. C. voltage and the impression of the I. F. signal may be accomplished in a variety of ways, as outlined in the above-mentioned copending application, besides the manner indicated herein.

Referring again to Fig. l, a system is illustrated which employs a traveling wave frequency converter for the stabilization of a microwave oscillator transmitted frequency. The converter-amplifier 1 has coupled to the radio frequency section 3 through the radio frequency input means 8 the output signal of a fixed frequency microwave oscillator to supply oscillatory energy at a frequency f1 for fiow along the slow-wave propagating electrode 9 to establish an electromagnetic field which interacts with a projected electron beam from the electron gun section 2 contained in the converter-amplifier 1. The intermediate frequency oscillatory source comprising an l. F. modulating signal source 16, a reactance tube 17, and an I. F. amplifier 18 modulates the projected electron beam at a frequency f2 by being applied to any of the beam forming electrodes 5, 6, and 7, or the helical electrode, in combinations mentioned hereabove through the network 13. The result of the converting and amplifying in the converter-amplifier 1, containing frequencies 1+fz, fi-fz, fi--Zfz, f1-2f2, and so forth, is coupled through the radio frequency output means 11 to the R. F. band pass filter 19 whose configuration is s uch that the frequency fi-l-fz is selected and coupled to the transmitting antenna 2G for transmission.

The transmitted frequency Ji-l-fz is monitored preferably by an R. F. reference .cavity discriminator 21, which is centered around the frequency f1-H2. When the transmitted frequency drifts from the center frequency, f1|f2, there is an error voltage fed back along the feedback path 22 to the reactance tube 17 in such a manner that the modulating frequency to the converter-amplifier 1 is corrected to return the transmitted frequency to the center frequency. Selection of the intermediate frequency and its bandwidth must be such that it is larger than the frequency drift of the microwave oscillator to establish an effective stabilization system.

Fig. 2 illustrates the adaptation of a converter-amplifier as herein described for employment as a traveling wave tube oscillator and the associated circuitry for frequency stabilization. The converter-amplifier 1 has associated with its radio frequency input coupling means a feedback path provided by the sharply-tuned radio frequency cavity 34, tuned at frequency f1. When the converter-amplifier is in operation, a predetermined amount of its output is fed back through the radio frequency cavity 34 which resonates at frequency f1 establishing the radio frequency oscillatory energy for travel along the slow-wave propagating electrode contained in the converter-amplifier 1. The oscillatory modulation signal is supplied by the intermediate oscillator 35 and the reactance tube 36. The resonant frequency output will be coupled through band pass filter 19 to the transmitting antenna 37 for transmission. 'I'he radio frequency reference cavity discriminator 38 is centered at the frequency fi+ fm and monitors the output of the radio frequency band pass filter 19. Any frequency error that develops produces an error voltage from the discriminator 38 which is passed along the feedback path 39 to the reactance tube 36 and hence to the oscillator 35 for control of the intermediate oscillatory frequency. This altered oscillatory signal may be applied between any of the two electrodes contained in the electron gun of the converter-amplifier 1 or any combination of such modulation points as herebefore mentioned to produce the desired amount of phase shift of the electrons in the beam to lock the traveling wave tube oscillator at a fixed frequency. A combination of the 'discriminator 38 and the sharply-tuned R. F. cavity 34 herein described produces a traveling wave tube oscillator which is well stabilized and contains a high level power output;

Fig. 3 illustrates another type of terminal transmitter adaptable for microwave repeaters. Herein the variable frequency oscillator 27 is frequency stabilized by employing a feedback loop incorporating an R. F. reference cavity discriminator 28 tuned to a single frequency f1. This fixed frequency from the oscillator 27 is applied to the radio frequency section 3 through the R. F. input means 8 for travel down the slow-wave propagating electrode 9 and for interaction with the closely associated electron beam of the converter-amplifier 1 substantially as discussed in connection with Fig. 1. The electron beam is modulated directly from the sub-carrier system 29 stabilized by the discriminator 30 having a suitable time constant. This sub-carrier system 29 is modulated by the video modulating frequency from signal" source 31 through the reactance tube 32. The modulated output of sub-carrier 29 modulates the electron beam by being impressed to the electrodes of the converter-amplifier 1, as discussed in connection with Figs. 1 and 2. The output from the converter-amplifier 1 is coupled to the R. F. band pass filter 19 whose configuration is such that the desired side band is selected for transmission from the transmitting antenna 33. A terminal transmitter system as herein described will now also be inherently wide band when good linearity is achieved and capable of high level power outputs.

Fig. 4 illustrates a terminal transmitter adaptable for avzmsaa and'hence to the antenna. Thisscheme is inherently narrow band when goodlinearity is achieved. However, in the system shown in Fig.,2,the variable frequency microwave oscillator 23, either a klystron or magnetron oscillator,` isV frequency stabilized by employing a feedback loop vincorporating a radio frequency reference cavity discriminator 24 tunedv to a single frequency, fi. This fixed microwave frequency supplies the oscillatory source for travel along the slow-wave propagating electrode and-interaction with the electron beam of the converter-amplifier 1, having substantially the same structure and arrangement as that shown in Fig. l. The modulation of the traveling wave. converter-amplifier 1 is accomplished by an. oscillatory modulation signal fm from a video modulator orsignal source 2S through a network 26, the output of which isinversely proportional to frequency. This output from network 26 may then be applied to any one of the electrodes,.including the helical electrode, or combinations` thereof, as described in connection with Fig. l, producing a modulation index that varies with the modulating frequency fm in the manner required for an FM signal. The desired side-band frequency for transmission is selected bythe radio frequency filter 19. Thus a system isl developed which allows for direct video modulation of the microwave frequency from oscillator 23 and which is now inherently wide band'and capable of4 a high power output.

By substitution of a low frequency electron device producing low frequency noise for the network 26and'video modulator 25 in the system of say Fig. 4, it is possible to obtain anoise. source in the microwave region. This microwave noise source will be achieved substantially the same as the stabilized'oscillator signal above with the 'exception' thatthe resulting signal will be an. amplified noise. in the.V microwave region havinga bandwidth equal to .thatoffthe low frequency noisesource. Fig. 5 illustrates the adaption of a'converter-amplifer as herein described, for employmentV as a traveling wave tube oscillator. and the. associated circuitry forV frequency stabilization employing. afeedback loop for altering the microwave, source.4 The convertereampliier I has associated with its radio frequency input coupling means the output of a microwave oscillator 40 establishing the radio frequency oscillatory energy for travel along the slowwave propagating electrode contained therein. The intermediate oscillatory sourcer comprises an I. F. crystal controlled oscillator 42 and a signal source 44, the output from the oscillator 42 modulating the projected electron beam at a frequency f2 as herebefore described. The frequency obtained in the converting process will be coupled through the band pass filter 19 to the transmitting antenna 37 for transmission. The radio frequency reference cavity discriminator 41 is centered at the frequency ful-fz and monitors the output of the radio frequency band pass filter 19. Any frequency error that develops produces an error voltage from discriminator 41 which is coupled to the microwave oscillator 40 along the error voltage path 43 for control of the radio frequency oscillatory frequency. This altered oscillatory signal may be applied between any of the two electrodes contained in the electron gun of the converter-amplifier 1 or any combination of such modulation points as herebefore mentioned for the desired amount of phase shift to provide a locking of the traveling wave tube oscillator at a xed frequency. The employment of ldscriminator 41 having a suitable time constant produces a traveling wave tube oscillator which is well stabilized and contains a high level power output.

While we have described above the principles of our invention in connection with specific apparatus, it is to v 6 beclearly understood that this description is'made'. only by way of example and not as a limitation tol the scope of our invention, asset forth inthe objects thereof and inthe accompanying claims.v

We claim: l l. A stabilizingsystem comprising two sources ofgoscilv'latory energy,` a converter-amplifier comprising a radio frequency electrode to dene a slow-wave axialy path,

means to'- apply the oscillatory energy from one: of' said sourcesv to said radio frequency'electrode for ow along said path, means tov produce an electron beam for flow along, said ,axial path for interaction with theelectromagnetic field produced by the energy flowing along said electrode and means forward. of said radio frequency electrode for modulating said beam with energy from the other of said sources,.means for stabilizing the signal output of. said converter-amplifier and output means' for removing modulatedL energyfromsaid path, said4k means for stabilizing the signal output includingl discriminator means coupled between said output means: and" ati least one of` said sources to apply thereto an error voltage;

2. A ,stabilizingsystem' according'to` claim l, wherein said means for stabilizing comprises a feedback loop which includes a radio frequency oscillatory source as said one of said sources and said discriminator means, saidV discriminatormeans being responsive to a frequency-error in said signal output to alter theoutput of said radio frequency oscillatory source for'cornpensation of said error.

3. A stabilizing system according to claim 1, wherein said meansfor'stabilizing comprisesa feedback loopwhich includes an intermediate frequency oscillatorysource as said other ofsad sources and said'discriminator means, said discriminator means beingresponsive toa frequency errorr ine saidf signal output to alter the output'of" said intermediate frequency oscillatory source forV compensation of said error. l

4. A- stabilizingl systenr'according to claim' l, wherein 'said' means for stabilizing comprises a feedback loop in- `cluding anV intermediatefrequency oscillatory source las 'saidintermediate frequency oscillatory source to compensate for said'error. 5.' AA stabilizingsystem according to claim' l, wherein saidlmeans'for: stabilizing further comprises a feedback loop' including afsharply-'tunedicavity as saidv` one offsaid sources, said converter-amplifier and a conductive path coupling the said output means to said sharply-tuned cavity, said sharply-tuned cavity being responsive to en ergy emitted from said output means to establish a radio frequency source.

6. A stabilizing system according to claim 5, wherein said means for stabilizing further includes a second feedback loop including an intermediate frequency oscillatory source as said other of said sources, said converter-amplier and said discriminator means, said discriminator means being responsive to a frequency error present in the signal output of said output means for altering the output of said intermediate frequency oscillatory source to compensate for said error.

7. A stabilizing system according to claim l, wherein said means for stabilizing comprises a feedback loop in cluding a radio frequency oscillatory source a-s said one of said sources, said converter-amplifier and -said discriminator means, said discriminator means being responsive to a frequency error present in the signal output of -said converter-amplier for altering the output of said radio frequency oscillatory source .to compensate for said error.

8. A frequency stabilized microwave oscillator comprising a fixed frequency microwave oscillator, a converter-amplifier including a radio frequency electrode coupled to said oscillator to define a slow-wave axial path,

means to produce an electron beam for flow along said axial path for interaction with the electromagnetic field vproduced by the energy Vflow along said path, an intermediate frequency oscillatory section to modulate said beam coupled to said electron beam producing means, an

`output means for extracting signal energy from said path,

a radio frequency band pass filter coupled to said output means to select the desired side band frequency output `therefrom for transmission, and a feedback loop comprising said converter-amplifier, a radio frequency discriminator, an error voltage path connecting said discriminator to said intermediate frequency section, and means coupling said filter tosaid discriminator for monitoring said side section to compensate for said variations.

9. A frequency stabilized traveling wave oscillator com- .prising a radio `frequency oscillatory source including a sharply-tuned resonant cavity, a converter-amplifier including an RF electrode coupled to said radio frequency source to define a slow-wave axial path, means to produce an electron beam for ow along said axial path for interaction with the electromagnetic field produced by the energy iiow along said path, an intermediate frequency oscillatory section to modulate said beam coupled to said electron beam producing means, output means for extracting signal energy from said path, a radio frequency band pass filter coupled to said output means to select the desired side band frequency output therefrom for transmission, a feedback path from said output means to excite said radiofrequency source with arfraction of said signal energy, and a feedback loop comprising said converter-amplifier, a radio frequency discriminator, an error voltage path connecting said discriminator to said intermediate frequency section, and means coupling said filter to said discriminator for monitoring said side band frequency output, variations therein producing an error voltage for conduction along said error voltage path for altering the output of said intermediate frequency section to compensate for said variations.

10. A frequency stabilized traveling wave oscillator comprising a microwave oscillator, a converter-amplifier including a radio frequency electrode coupled to said oscillator to define a slow-wave axial path, `means to produce an electron beam for ow along said axial path for interaction with the electromagnetic field produced by the energy ow along said path, an intermediate frequency oscillatory section comprising a signal source coupled to a crystal controlled intermediate frequency oscillator whereby the output therefrom is coupled to said electron Vbeat-ln producing means to modulate said beam, an output means for extracting signal energy from said path, a radio frequency band pass 'filter coupled to said output means to select the desired side band frequency output r'therefrom for transmission, and a feedback loop compris- Ving said converter-amplifier, a radio frequency discriminator, an error voltage path connecting said discriminator to Asaid microwave oscillator, and means coupling said lter to said discriminator for monitoring said side band fre- ,quency output, variations therein producing an error "voltage forconduction along said error voltage path for altering the output of said microwave oscillator to compensate for said variations.

11. A microwave noise source comprising a variable frequency microwave oscillator, a converter-amplifier including a radio frequency electrode coupled to said oscillator to define a slow-wave axial path, means to produce an electron beam for ow along said axial path for interaction with the electromagnetic eld produced by the energy ilow along said path, an intermediate frequency .oscillatory section comprising a low frequency electron device producing a noise output coupled to an intermediate frequency amplifier whereby the output therefrom is coupled to said electron beam producing means to modulate said beam, output means for extracting signal energy from said beam, a radio frequency band pass filter coupled to said output means to select the desired noise side band frequency output therefrom for transmission, and a feed back loop consisting of a radio frequency reference cavity and the said oscillator providing means to stabilize the 4radio frequency oscillatoryenergy coupled to said radio frequency electrode.

References Cited in the le of this patent UNITED STATES PATENTS Schultz May 26, 1953 

